The present invention is with respect to film evaporators and more specially to such an evaporator made up of a central turning driving shaft, spaced conical surface elements joined to said driving shaft with an evaporation space within them and a heat vehicle space placed round said evaporation space on the outside of the surface elements, the product being run into the evaporator at inner edges of the conical surface elements so that it is then spread out in the form of a film and is run off from the evaporator after being heated by the heat vehicle.
A number of different designs of film evaporators using turning conical evaporation surface elements are to be seen in the German Offenlegungsschrift specifications Nos. 2,510,206, 2,603,480, and 2,951,689. Such evaporators outdo other designs inasfar as the product is spread out into a thin, film-like layer so that, more specially when the evaporator is run under vacuum, chemical, pharmaceutical and foodstuff products may be concentrated within a short residence time and liquids driven off therefrom with a minimum of undesired effects on product quality. In the known designs there is a single conical evaporation surface of the desired size, or there may be a battery of evaporation elements placed one after the other on the driving shaft so that there is an increase in the evaporation rate.
In the case of such evaporators the speed of turning the shaft, the rate of feed of the product into the apparatus and the temperature gradient on the evaporator surface have to be in so controlled and have such a relation to each other that a full and unbroken film is kept up all over the evaporation surface. To keep up such an unbroken film, the film thickness then has to be markedly greater where the material is run into the apparatus at the inner edge of the evaporation face than further out, because the evaporation area of a given amount of film becomes greater with the square of its distance from the center of the shaft. Under likely working conditions it is frequently not possible to keep to this condition, more specially because the system is highly sensitive to changes in the viscosity and the surface tension of product and furthermore to the concentration ratio (condensing ratio). Once there is a break in the film, the bare evaporation surface is likely to be overheated at this point with the outcome that the product may be denatured. Incrustations may be formed or the rate of evaporation may go down. This danger is more specially great in the case of heavy concentration of weak solutions and of distilling solutions, with a low impurity concentration, of organic substances of synthetic or natural origin.